Advances in Pure Mathematics
Vol.3 No.6(2013), Article ID:36373,13 pages DOI:10.4236/apm.2013.36072
Nemytskii Operator in the Space of Set-Valued Functions of Bounded -Variation
Departamento de Fsica y Matemática, Universidad de Los Andes, Trujillo-Venezuela, RBV
Email: wadie@ula.ve
Copyright © 2013 Wadie Aziz. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Received June 13, 2013; revised July 23, 2013, accepted August 19, 2013
Keywords: Bounded Variation; Function of Bounded Variation in the Sense of Riesz; Variation Space; Weight Function; Banach Space; Algebra Space
ABSTRACT
In this paper we consider the Nemytskii operator, i.e., the composition operator defined by, where
is a given set-valued function. It is shown that if the operator
maps the space of functions bounded
-variation in the sense of Riesz with respect to the weight function
into the space of set-valued functions of bounded
-variation in the sense of Riesz with respect to the weight, if it is globally Lipschitzian, then it has to be of the form
, where
is a linear continuous set-valued function and
is a set-valued function of bounded
-variation in the sense of Riesz with respect to the weight.
1. Introduction
In [1], it was proved that every globally Lipschitz Nemytskii operator
mapping the space into itself admits the following representation:
where is a linear continuous set-valued function and
is a set-valued function belonging to the space
. The first such theorem for singlevalued functions was proved in [2] on the space of Lipschitz functions. A similar characterization of the Nemytskii operator has also been obtained in [3] on the space of set-valued functions of bounded variation in the classical Jordan sense. For single-valued functions it was proved in [4]. In [5,6], an analogous theorem in the space of set-valued functions of bounded
-variation in the sense of Riesz was obtained. Also, they proved a similar result in the case in which that the Nemytskii operator N maps the space of functions of bounded
-variation in the sense of Riesz into the space of set-valued functions of bounded
-variation in the sense of Riesz, where
, and
is globally Lipschitz. In [7], they showed a similar result in the case where the Nemytskii operator
maps the space
of setvalued functions of bounded
-variation in the sense of Riesz into the space
of set-valued functions of bounded
-variation in the sense of Riesz and
is globally Lipschitz.
While in [8], we generalize article [6] by introducing a weight function. Now, we intend to generalize [7] in a similar form we did in [8], i.e., the propose of this paper is proving an analogous result in which the Nemytskii operator maps the space
of setvalued functions of bounded
-variation in the sense of Riesz with a weight
into the space
of set-valued functions of bounded
-variation in the sense of Riesz with a weight
and
is globally Lipschitz.
2. Preliminary Results
In this section, we introduce some definitions and recall known results concerning the Riesz -variation.
Definition 2.1 By a -function we mean any nondecreasing continuous function
such that
if and only if
, and
as
.
Let be the set of all convex continuous functions that satisfy Definition 2.1.
Definition 2.2 Let be a normed space and
be a
-function. Given
be an arbitrary (i.e., closed, half-closed, open, bounded or unbounded) fixed interval and
a fixed continuous strictly increasing function called a it is weight. If
, we define the (total) generalized
-variation
of the function
with respect to the weight function
in two steps as follows (cf. [9]). If
is a closed interval and
is a partition
of the interval I (i.e.,
), we set
Denote by the set of all partitions of
, we set
If is any interval in
, we put
The set of all functions of bounded generalized - variation with weight
will be denoted by
.
If, and
,
,
, the
-variation
, also written as
, is the classical
-variation of
in the sense of Riesz [10], showing that
if and only if
(i.e.,
is absolutely continuous) and its almost everywhere derivative
is Lebesgue
-summable on
. Recall that, as it is well known, the space
with I,
and
as above and endowed with the norm
is a Banach algebra for all.
Riesz’s criterion was extended by Medvedev [11]: if, then
if and only if
and. Functions of bounded generalized
-variation with
and
(also called functions of bounded Riesz-Orlicz
-variation) were studied by Cybertowicz and Matuszewska [12]. They showed that if
, then
and that the space
is a semi-normed linear space with the LuxemburgNakano (cf. [13,14]) seminorm given by
.
Later, Maligranda and Orlicz [15] proved that the space equipped with the norm
is a Banach algebra.
3. Generalization of Medvedev Lemma
We need the following definition:
Definition 3.1 Let be a
-function. We say
satisfies condition
if
(1)
For φ convex, (1) is just. Clearlyfor
the space
coincides with the classical space
of functions of bounded variation. In the particular case when
and
, we have the space
of functions of bounded Riesz
-variation. Let
be a measure space with the Lebesgue-Stieltjes measure defined in -algebra
and
Moreover, let be a function strictly increasing and continuous in
. We say that
has
- measure 0, if given
there is a countable cover
by open intervals of
, such that
.
Since is strictly increasing, the concept of “
measure
” coincides with the concept of “measure 0” of Lebesgue. [cf. [16],
25].
Definition 3.2 (Jef) A function is said to be absolutely continuous with respect to
, if for every
, there exists
such that
for every finite number of nonoverlapping intervals,
with
and
.
The space of all absolutely continuous functions, with respect to a function
strictly increasing, is denoted by
. Also the following characterization of [17,18] is well-known:
Lemma 3.3 Let. Then
exists and is finite in
, except on a set of
-measure
.
Lemma 3.4 Let. Then
is integrable in the sense Lebesgue-Stieltjes and
Lemma 3.5 Let such that satisfies the
condition. If, then
is
-absolutely continuous in
, i.e.,
Also the following is a generalization of Medvedev Lemma [11]:
Theorem 3.6 (Generalization a Medvedev Lemma) Let such that satisfies the
condition,
. Then 1) If
is
-absolutely continuous on
and
then
and
.
2) If (i.e.,
), then
is
-absolutely continuous on
and
.
Proof.) Since
is
absolutely continuous, there exists
a.e. in
by Lemma 3.3. Let
,
by Lemma 3.4 and is strictly increasing
using the generalized Jenssen’s inequality
Let be any partition of interval
; then
and we have
.
Thus.
) Let
. Then
is
-absolutely continuous on
by Lemma 3.5 and
exist a.e. on
.
For every, we consider
a partition of the interval define by
,
.
Let be a sequence of step functions, defined by
converge to
a.e. on
. It is sufficient to prove
in those points where
is
- differentiable and different from
,
for
, i.e., in
For, and each
, there exists
such that
, so
Therefore, is a convex combination of points
Now if, then
and
and since
is
-differentiable for
, the expressions
tend to which is
-differentiable from
in
. So results
Since is continuous, we have
Using the Fatou’s Lemma and definition of sequence, results that
By definition from
which is what we wished to demonstrate.
Corollary 3.7 Let such that satisfies the
condition, then
if and only if
is
-absolutely continuous on
and
.
Also
Corollary 3.8 Let such that satisfies the
condition. If
, then
is
-absolutely continuous on
and
4. Set-Valued Function
Let be the family of all non-empty convex compact subsets of
and
be the Hausdorff metric in
, i.e.,
where, or equivalently,
where
(2)
Definition 4.1 Let,
a fixed continuous strictly increasing function and
. We say that
has bounded
-variation in the sense of Riesz if
(3)
where the supremum is taken over all partitions of
.
Definition 4.2 Denote by
(4)
and
(5)
both equipped with the metric
(6)
where
Now, let,
be two normed spaces and
be a convex cone in
. Given a set-valued function
we consider the Nemytskii operator
generated by
, that is the composition operator defined by:
We denote by the space of all setvalued function
, i.e., additive and positively homogeneous, we say that
is linear if
.
In the proof of the main results of this paper, we will use some facts which we list here as lemmas.
Lemma 4.3 ([19]) Let be a normed space and let
be subsets of
. If
are convex compact and
is non-empty and bounded, then
(7)
Lemma 4.4 ([20]) Let,
be normed spaces and
be a convex cone in
. A set-valued function
satisfies the Jensen equation
(8)
if and only if there exists an additive set-valued function and a set
such that
,
.
We will extend the results of Aziz, Guerrero, Merentes and Sánchez given in [8] and [21] to set-valued functions of -bounded variation with respect to the weight function
.
5. Main Results
Lemma 5.1 If such that satisfies the
condition and
then
is continuous.
Proof. Since, exists
such that
(9)
for all partitions of, in particular given
, we have
(10)
Since is convex
-function, from the last inequality, we get
(11)
By (1),
(12)
This proves the continuity of at
. Thus
is continuous on
.
Now, we are ready to formulate the main result of this work.
Main Theorem 5.2 Let,
be normed spaces,
be a convex cone in
and
be two convex
-functions in
, strictly increasing, that satisfy
condition and such that there exists constants
and
with
for all
. If the Nemitskii operator
generated by a set-valued function
maps the space
into the space
and if it is globally Lipschitz, then the set-valued function
satisfies the following conditions:
1) For every there exists
, such that
(13)
2) There are functions and
such that
(14)
Proof. 1) Since is globally Lipschitz, there exists a constant
such that
(15)
Using the definitions of the operator and metric
we have
where. In particular,
for all and
,
, where
.
Since and
satisfy
(16)
we obtain
Therefore
(17)
Define the auxiliary function by:
(18)
Then and
Let us fix and define the functions
by:
(19)
Then the functions
and
(20)
From the definition of and
, we have
(21)
From (16), we get
(22)
Hence,
(23)
Hence, substituting in inequality (5) the particular functions
defined by (19) and taking
in (23), we obtain
(24)
for all.
By Lemma 4.3 and the inequality (24), we have
for all.
Now, we have to consider the case. Define the function
by
(25)
Then the function and
Let us fix and define the functions
by
(26)
Then the functions (i = 1,2) and
Substituting and
, and consider
, we obtain
(27)
for all, where
By Lemma 4.3 and the above inequality, we get
for all. Define the function
by
Hence
and, consequently, for every the function
is continuous.
This completes the proof of part 1).
Now we shall prove that satisfies equality 2).
Let us fix such that
. Since the Nemytskii operator
is globally Lipschitzian, there exists a constant
, such that
(28)
where. Define the function
by
The function.
Let us fix and define the functions
by
(29)
The functions
and
.
Hence, substituting in the inequality (28) the particular functions
defined by (29), we obtain
(30)
Since maps
into
then
is continuous for all
. Hence letting
in the inequality (30), we get
(31)
for all and
.
Thus for all, we have
(32)
Since is convex, we have
(33)
for all. Thus for all
, the set-valued function
satisfies the Jensen Equation (33). Now by Lemma 4.4, there exists an additive set-valued function
and a set
, such that
(34)
Substituting into inequality (13), we deduce that for all
there exists
, such that
consequently, for every the set-valued function
is continuous, and
.
Since is additive and
, then
for all
, thus
.
The Nemytskii operator maps the space
into the space
, then
.
Consequently the set-valued function has to be of the form
where and
.
Theorem 5.3 Let,
be normed spaces,
a convex cone in
and
be two convex
-functions in
, strictly increassing, satisfying
condition and. If the Nemytskii operator
generated by a set-valued function
maps the space
into the space and if it is globally Lipschizian, then the set-valued function
satisfies the following condition
i.e., the Nemytskii operator is constant.
Proof. Since the Nemytskii operator is globally Lipschizian between
and the space
, then there exists a constant
, such that
(35)
Let us fix such that
. Using the definitions of the operator
and of the metric
, we have
(36)
Define the auxiliary function by
The function and
Let us fix and define the functions
by
(37)
The functions
and
Hence, substituting in the inequality (36) the auxiliary functions
defined by (37), we obtain
By Lemma 4.3 and the above inequality, we get
Since, letting
in the above inequality, we have
Thus for all and for all
, we get
Theorem 5.4 Let,
be normed spaces,
a convex cone in
and
be a convex
- function in
satisfying the
condition. If the Nemytskii operator
generated by a set-valued function
maps the space
into the space
and if it is globally Lipschizian, then the left regularization
of the function
defined by
satisfies the following conditions:
• for all there exists
, such that
•
• , where
is a linear continuous set-valued function, and
.
Proof. We take, and define the auxiliary function
by:
The function and
Let us fix and define the functions
by
(38)
The functions and
(39)
From the definition of and
, we obtain
(40)
Since the Nemytskii operator is globally Lipschitzian between
and
then there exists a constant
, such that
for. By Lemma 4.3, substituting the particular functions
defined by (38) in the above inequality, we obtain
(41)
for all. By Lemma 4.3, we get
(42)
for all and
.
In the case where, by a similar reasoning as above, we obtain that there exists a constant
, such that
(43)
Define the function by
(44)
Hence,
By passing to the limit in the inequality (41) by the inequality (43) and the definition of we have for all
that there exists
, such that
Now we shall prove that satisfies the following equality
where is a linear continuous set-valued functions, and
.
Let us fix such that
. Define the partition
of the interval
by
The Nemytskii operator is globally Lipschitzian between
and
, then there exists a constant
, such that
(45)
where
and
.
We define the function in the following way:
The function and
.
Let us fix and define the functions
by:
(46)
The functions and
.
Substituting in the inequality (45) the particular functions
defined in (46), we obtain
(47)
Since the Nemytskii operator maps the spaces
into
, then for all
, the function
. Letting
in the inequality (47), we get
for all and
. By passing to the limit when
, we get
Since is a convex function, then
Thus for every, the set-valued function
satisfies the Jensen equation. By Lemma 4.4 and by the property (a) previously established, we get that for all
there exist an additive set-valued function
and a set
, such that
By the same reasoning as in the proof of Theorem 5.2, we obtain that
and
.
6. Acknowledgements
This research was partly supported by CDCHTA of Universidad de Los Andes under the project NURR-C- 547-12-05-B.
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